Take for example a GetUserStatistics() call which provides a list of userids and the users last login date.

A client might be using this list to get statistics on system usage.

If you change the codebase to add the concept of a test user and add an isTestUser column to GetUserStatistics() you have broken the contract with your users.

You had an implicit contract based on shared understanding of the data.

Now of course to correctly determine user usage statistics you need to exclude the test users by checking the new column.